It is known that so-called "acid gases", i.e. gaseous media containing acid forming substances, such as sulphur dioxide, hydrogen chloride and the like, can be cleansed of their acid constituents, by washing such gases with water containing alkaline components. Although this method provides good results, however, it is encumbered with high investment costs and the apparatus required to carry out the method must be carefully maintained.
It is also known to provide such apparatus with advanced water and sludge treatment auxiliaries, otherwise it would be immediately ascertained that an air contaminant has transformed to a water contaminant.
One reason for the high investment costs incurred is that the whole purification plant must be constructed from high-grade stainless steel, since acidiferous aqueous solutions are extremely aggressive.
It is also known to employ so-called "semi-dry" purification systems or "fully dry" purification systems, in an attempt to eliminate the aforesaid drawbacks.
In methods used in conjunction with such purification plants there is injected into the acidic medium a pulverulent absorbent which presents a large specific surface area and which is reactive with the acid components of the medium, e.g. a flue gas, so that the acid components as a result of this reaction adhere to and absorb on the discrete particles of the pulverulent absorbent. The absorbent material carrying the contaminants can then readily be separated from the medium by conventional methodology, with the aid of electrostatic separators and/or fabric filters.
It is well known that these dry purification systems functioning by absorption afford many advantages compared with known wet separation methods, inter alia because the contaminated medium, flue gases, is treated with a dry absorbent and at high temperatures. The use of a dry absorbent in hot flue gases has enabled standard constructional steel to be used in the construction of the plant apparatus required. This has greatly reduced construction costs.
Maintenance costs, personnel costs and running costs are also lower when practicing this latter type of purification methodology.
It has been found, however, that the reaction between an acid gas component, for example gaseous hydrogen chloride, and a dry solid particle, for example slaked lime Ca(OH).sub.2, is not as effective as the reaction between hydrogen chloride gas and a basic water droplet.
It has also been established that in a dry purification system reactivity can be enhanced by decreasing the particle size of the dry particles. This renders the absorbent more expensive however.
The arrangements and methods described and illustrated in
(a) British Patent Specification No. 941 618, and PA0 (b) Swedish Patent Specification No. 422 890 PA0 (a) removing downstream of the contact reactor all or a part of the absorbent which has partly reacted with the contaminants; PA0 (b) storing all or part of the said removed material for a period of time in excess of 20 hours; PA0 (c) causing the contaminants to diffuse inwardly into the individual particles during this storage period, thus increasing the absorption ability of the particle surface; and PA0 (d) charging the thus stored, previously used absorbent to the contact reactor in the form of a pulverulent absorbent material capable of reacting with contaminants present in the medium.
also form part of the prior art.
The British Specification No. 941 618 describes and illustrates a plant in which fresh reactant is mixed with varying amounts of previously used and thus reacted material. Although this latter material is said to be taken from a silo, the specification makes no mention of any specific length of time over which the reacted material shall be stored in the silo.
The sole purpose of this storage silo is one of smoothing out variations in material flow due to fluctuations in operation, start and stop, and to disturbances which result in losses of material.
The Swedish Patent Specification No. 422 890 describes and illustrates a plant of principally similar operational features with regard to the storage and recycling of reactants which have previously undergone a reaction, similar to the aforesaid British Patent Specification.